Deciphering the Intersystem Crossing in a Helical BODIPY for Recorded Low Dose Photodynamic Therapy

: Intersystem crossing (ISC) of triplet photosensitizers is a vital process for fundamental photochemistry and photodynamic therapy (PDT). However, the currently effective long wavelength operating photosensitizers have to rely on heavy atoms to enhance ISC. Unfortunately, the heavy atoms, besides their toxicity, can also cause significant reduction in the triplet lifetime of the photosensitizers, which is fundamentally detrimental to their applications. To overcome this challenge, herein we report the co-existence of efficient ISC and long triplet excited lifetime in a heavy atom-free Bodipy helicene molecule. Via advanced theoretical computation and time-resolved electron paramagnetic resonance spectroscopy, we confirmed the ISC of the Bodipy is resulted from its twisted molecular structure and reduced symmetry. The twisted Bodipy shows exceptionally intense long wavelength absorption (e = 1.76 × 10 5 M -1 cm -1 at 630 nm), satisfactory triplet quantum yield (Φ T = 52%) and long-lived triplet state (τ T = 492 μs), leading to unprecedented performance as a triplet photosensitizer for PDT. Moreover, nanoparticles constructed with such helical Bodipy show surprisingly efficient PDT mediated antitumor immunity amplification with ultra-low dose (0.25 μg/kg body weight), which is several hundred times lower than that of the existing PDT reagents. Our study paves the way to design the next generation of triplet photosensitizers.